Flexible electrical heating devices



June 4, 1968 I 0. E. w. REES 3,387,248

FLEXIBLE ELECTRICAL HEATING DEVICES Filed April 23, 1965 INVENTOR.0ona/a. m Rees HTTORNE Y United States Patent 3,387,248 FLEXIBLEELECTRICAL HEATING DEVECES Donald E. W. Rees, Barry, Glamorgan, Wales,assignor to Midland Siiicones Limited, Reading, England Filed Apr. 23,1965, Ser. No. 450,306 Claims priority, application Great Britain, May4, 1964, 18,346/ 64 3 Claims. (Cl. 338211) ABSTRACT OF THE DESCLOSURE Aflexible electrical heating device wherein the electrodes are bonded toa thin, flexible layer of electrically conducting silicone rubberemploying an electrically conducting, room-ternperature vulcanizingsilicone rubber to eflect electrical and physical connection andadhesion be tween the electrically conducting siilcone rubber and theelectrodes.

This invention relates to electrical heating elements, and moreparticularly to electrical heating elements of the type comprising asheet or tape material having conducting properties and having attachedthereto metallic or conducting silicone rubber electrodes.

Organopolysiloxanes and compositions based thereon are, in general, goodelectrical insulators and this property has led to the wide use of thesematerials as such in the electrical industry. It is also known to renderorganopolysiloxane compositions electrically conducting by incorporatingtherein a conducting filler such as a powdered metal or a carbon black.This proposal has made possible the development of electricallyconducting rubbers which are capable of operating at relatively hightemperatures and one example of an application where such a property maybe exploited is in the manufacture of flexible heating elements. Whenthe heating element comprises a substantial thickness of conductingsilicone rubber little difficulty is experienced in attaching theretothe connections necessary for app ying the activating source ofelectrical energy. However, when the flexible heating element comprisesa woven sheet of glass or other non-conducting fabric coated with a thinlayer of conducting rubber, or comprises a thin sheet of the conductingrubber, it is not easy to attach electrodes thereto in order that therequired voltage may be applied. For example, difliculty is experiencedin making a good electrical contact with the conducting layer that willwithstand a reasonable amount of physical stress, vibration andtemperature variation and yet not detract from the compact andlightweight nature of the device.

An object of the present invention is to provide a flexible heatingdevice having improved electrode attachment means.

A further and more specific object is the provision for flexible heatingelements of improved electrode attachment means capable of maintaininggood electrical contact while withstanding physical stresses, vibrationand temperature variation while remaining lightweight and compact.

In accordance with these and other objects there is provided by thepresent invention an electrode attachment means comprising anelectrically-conducting organopolysiloxane-based, cold setting adhesiveas a means whereby the electrode can be caused to adhere to the sheet orlayer of rubber and whereby electrical and physical connection is madebetween the flexible conducting member and the leads thereof.

Other objects and many of the attendant advantages of the presentinvention will become obvious to those skilled in the art by aconsideration of the following de- 3,387,248 Patented June 4, 1968tailed description when read in conjunction with the accompanyingdrawings wherein:

FIG. 1 is a view in perspective of an electrical heating element made inaccordance with the present invention; and

FIG. 2 is a cross-sectional view of the element shown in FIG. 1 andtaken along the line 22 of that figure.

Referring now to the drawings, wherein like reference charactersdesignate like parts throughout the views, there is shown in the figuresa flexible, electrically conducting device comprising a flexible layeror sheet 11 of an electrically conducting silicone elastomeric materialsaid layer or sheet having metal electrodes 12 and 13 attached theretoby means of an electrically conducting cold-setting, organopolysiloxanebased, adhesive composition 14, 15.

The conducting layer or sheet 11 of elastomeric material forming part ofthe device of this invention may be supported or unsupported, that is,it may comprise a layer or a coating of an electrically-conductingsilicone elastomer on a fibrous or woven, insulating support material 16or it may comprise only an unsupported sheet or tape of the saidelastomeric material. Preferably the conducting or resistive layercomprises a relatively thin coating of the elastomer on a fibroussupport which is, for example, woven glass or polyethylene terephthalatecloth. The completed element may be insulated as required with a coatingof insulation.

The conducting silicone elastomeric material of which the layer or sheetis comprised is based upon a vulcanizable organosilicon polymer.Preferably the organosilicon polymer is a high-molecular-weightdiorganopolysiloxane in which the organic substituents are selected fromlower a'lkyl, alkenyl and phenyl radicals. The general preparation ofelectrically conducting silicone elas tomers and their use in thefabrication of heating elements and in other electrical applications iswell-known.

The layer or sheet 11 of conducting or resistive material has attachedthereto metal electrodes or leads 12, 13 by means of which electricalcontact between the layer or sheet and an external source of electricalenergy is achieved. In one method of carrying out this invention theelectrodes are attached to the layer or sheet of conducting material byadhesion thereto employing a cold setting silicone rubber which has beenrendered electrically conducting by the incorporation therein of aquantity of a conducting or semiconducting filler. The electrode, orthat part of it which is in contact with the conducting layer, maycomprise a metal for example aluminium, foil or it may consist of amulti strand electrical conductor. Attachment of the electrode may takeplace by simple adhesion, the cold setting silicone rubber beingemployed as the adhesive. Alternatively the electrode may be embedded ina mass of the cold setting rubber which has been deposited on theresistive layer. Electrical contact between the electrode and the layerwill be ensured in view of the conducting nature of the cold settingrubber.

In order to improve the fixation of the electrode it may also beanchored mechanically to the sheet or layer, for example by interlacingit with the weave of the fibrous support, where one is employed, oralternatively crimping the electrode to the layer or sheet prior toembedding in the cold setting rubber.

The cold setting silicone adhesives employed in the practice of thisinvention may be any of those known in the art which are capable ofproducing a flexible bond when vulcanized and which may be loaded with asufficient proportion of a conducting filler, for example carbon blackor a finely divided metal such as silver, to endow the adhesive with therequired electrical properties. Such cold setting adhesives include forexample compositions based on acetoxy or oxirne substitutedorganopolysiloxanes and which vulcanize merely on exposure to moisture,and the so-called two component cold-curing systems comprising areactive siloxane polymer, a cross linking agent and a condensationcatalyst. Room temperature vulcanizing silicone rubbers are suitablewhen sufiiciently loaded with conducting filler.

In order to improve the strength of the vulcanized adhesive the coldsetting siloxane compositions may contain fillers, such as silicas andmetallic oxides, in addition to the filler or fillers required to givethe composition its conductive properties.

The following example illustrates the invention.

Example A tape one inch wide and having a thickness of 0.030 inch wasprepared by extruding and thereafter curing a composition comprising byweight 80 parts of a methylvinyl -polysiloxane gum, parts of a silicafiller and parts of a conducting carbon black. A cold curingorganopolysiloxane adhesive composition was prepared comprising amixture of adimethylpolysiloxane, an alkyl polysilicate and an organotincompound as condensation catalyst, there being added 25 parts by weightof carbon black for every 100 parts of the adhesive composition torender it electrically conducting. This composition was then applied tothe ends of a 12-inch length of the conducting tape and aluminium foilelectrodes of dimensions /2 x 1 inch held in contact with the adhesive.After several hours the electrodes had become firmly attached to thetape and the electrical resistance between the electrodes was 30,000ohms measured at 10 v. DC.

The device thus formed has a sustained high temperature capability andgood electrical contact is maintained even under physical stress,vibration and extremes in temperature variation. The device is compactand light in weight. The manufacturing process is simple and economical.

Obviously variations and modifications of the present invention, otherthan those described, will become obvious to those skilled in the art.It is therefore to be understood that within the scope of the appendedclaims the invention may be practiced otherwise than as specificallydescribed.

That which is claimed is:

1. A flexible electrically conducting heating device comprising:

a thin flexible layer of an electrically conducting silicone-elastomericmaterial, a pair of flexible metal electrodes in contact with andadhered to the surface of said silicone elastomeric material by a cured,electrically conducting, room-temperature vulcanized silicone rubber,said electrodes being arranged in approximate parallel relationship witheach other.

2. A flexible heating device as defined in claim 1 wherein the flexiblelayer consists of a fibrous support of an insulating material with acoating of a vulcanized electrically conductive silicone rubber.

3. A flexible heating device as defined in claim 1 wherein the flexiblemetal electrodes are metal foil.

References Cited UNITED STATES PATENTS 2,876,392 3/1959 Sanders 338-309X 2,982,934 5/1961 Browne 338-323 3,050,490 8/1962 Nitzscheet al. 260-373,060,303 10/1962 Skoglund et al 219-549 3,070,566 12/1962 Nitzsche etal 260-37 3,127,363 3/1964 Nitzsche et al. 260-18 3,287,684 11/1966Armbruster 338-211 3,296,195 1/ 1967 Goossens 260-465 2,559,077 7/ 1951Johnson et al 219-528 X 2,683,673 7/1954 Silversher 338-211 X 2,745,9315/1956 Heibel 338-203 2,781,277 2/1957 Dwyer 338-203 X 2,789,155 4/1957Marshall et al. 1'74-120 2,952,761 9/1960 Smith-Johannsen 117-226 X2,961,522 11/1960 Hammer 219-549 X 3,099,578 7/1963 Hunter 338-308 X3,179,544 4/ 1965 Smith-Johannsen 117-22 6 X 3,221,145 11/1965 Hager219-549 FOREIGN PATENTS 674,309 11/ 1963 Canada.

BERNARD A. GILHEANY, Primary Examiner. VOLODYMYR Y. MAYEWSKY, Examiner.

